The Long Term Evolution (LTE) Single-Carrier (SC) Frequency Division Multiple Access (FDMA), SC-FDMA, provides separate physical channels for the transmission of data and control signalling in uplink (UL), the latter being predominantly to support the downlink (DL) data transmissions. The detailed structure of these channels is designed to make efficient use of the available frequency-domain resources and to support effective multiplexing between data and control signalling.
The uplink transmission resources are defined in the frequency domain, with the smallest unit of resource being a Resource Element (RE), consisting of one SC-FDMA symbol length on one subcarrier. A Resource Block (RB) comprises 84 REs in the frequency domain for a duration of 1 slot and two RBs in same subframe (1 millisecond, ms, in time domain) form one RB pair which is often called a Physical Resource Block (PRB) pair.
There are three different physical channels in uplink, Physical Uplink Control Channel (PUCCH), Physical Random Access Channel (PRACH) and Physical Uplink Shared Channel (PUSCH). These channels share the frequency domain resources (PRB pairs) in uplink and if a certain PRB pair is used by one channel, it can not be used by other channels in a same subframe. Therefore, from an uplink capacity point of view, it is not desirable to statically allocate too many PRB pairs to PUCCH since it will reduce the number of PRB pairs which can be used by PUSCH. PRACH uses only a limited number of PRB pairs in a certain subframe, which is pre-defined by The Third Generation Partnership Project (3GPP), PUCCH and PUSCH are the channels that primarily matter when considering the uplink capacity.
A static allocation of PUCCH resource is often used, which makes it possible to manage uplink channels in a predictable manner. Also, it does not require any subsequent reconfigurations over Radio Resource Control (RRC).
There are three different formats for the PUCCH PRB pairs, i.e. format 1, format 2 and format 3, as defined in 3GPP Technical Specification (TS) 36.211 chapter 5.4. There are three different types of resources in the PUCCH format 1 PRB pairs, which are Hybrid Automatic Repeat Request (HARQ) acknowledgements (ACK) for the downlink transmissions, Scheduling Request (SR) and Channel Selection (CS). The PUCCH format 2 PRB pairs are used for Channel State Information (CSI) resources. The PUCCH format 3 PRB pairs are used for the HARQ acknowledgments for the downlink transmissions which requires multiple HARQ acknowledgments, e.g. in a 3 cell carrier aggregation case. In accordance with 3GPP there are certain rules for how to decide the position of each format. For example the position of the PUCCH format 1 PRB pairs is defined by the 3GPP so that it is on the inside of the PUCCH format 2 (CSI) PRB pairs. “Inside” here means located closer to the centre frequency of the bandwidth. PUCCH format 3 PRB pairs are then often placed on the inside of the PUCCH format 1 PRB pairs.